Study On The Characterization Of Principal Modes In Few-mode Optical Fibers

In the information age,the demand for communication capacity is growing rapidly,and as a solution to increase the communication capacity,the mode-division multiplexing(MDM)attracting much attention.Using principal modes(PMs)as the base of channel multiplexing is an important branch of the research on the MDM system.By using the frequency invariance of PMs in few-mode fibers(FMFs),we can effectively avoid the influence of cross-talk between modes,thus greatly reducing the complexity of signal processing and helping the implementation of large capacity real-time transmission scheme.At present,the characterizations of PMs mainly depend on the measurement of the propagation matrix of the optical fiber channels,which have a high cost of equipment and take a long time.Based on the study of the frequency invariance of PMs,this paper further proposes PMs'characterization method based on the spatial and spectral(S²)method,which is simple and cost-effective.This thesis mainly includes the following five aspects:(1)Based on the coupled modes equations theory,the propagation matrix of the optical fiber was obtained,which laid the foundation for the subsequent scheme simulation.(2)The basic principle of PMs was analyzed.After Taylor expansion of the fiber transmission matrix,it was found that the high-order term is the key to affect the frequency invariance of PMs.(3)The simulation model of S² method was established,and the feasibility of using this method to represent PMs was proved in theory.This paper proposes a method to recover the original mode patterns by calculating the interference facula without the main-power mode.(4)The relationship between coupling coefficients and their fluctuation and frequency invariance was explored.It was found that the influence of coupling coefficients fluctuation is more intense,and the feasibility of using spatial and spectral method to characterize PMs is verified.(5)The experimental system of spatial and spectral method was set up to measure the four-mode fiber in natural state and pressurized state.It was found that the group delay peaks of LP_11a and LP_11b were separated under pressure,which is consistent with the simulation results under coupled condition.To sum up,this thesis clarified the influence of coupling coefficients fluctuation on the frequency invariance of PMs,verified the feasibility of applying the spatial and spectral method to the characterization of PMs,and provided an important reference for the practical application of PMs.